Daily ✦ Arcane, Conjuration, Fire, ImplementStandard Action Ranged 10
Flame Sphere:
Effect: You conjure a Medium flaming sphere that occupies a
square within range, and the sphere
attacks. Any creature that starts
its turn adjacent to the sphere takes
1d4 + Intelligence modifier fire
damage. As a move action, you can
move the sphere 6 squares.Target: One creature adjacent to the flaming sphereAttack: Intelligence vs. ReflexHit: 2d6 + Intelligence modifier fire damage.Sustain Minor: You can sustain this power until the end of the
encounter. As a standard action, you
can make another attack with the
sphere

If flame sphere is created in a square adjacent to a prone enemy, does it gain combat advantage since it's making a melee attack against a prone target?

Given this, and the other question posted regarding this attack, I think this power (and others like it) may be slightly broken in terms of rules verbiage. I suggest you submit these issues to WotC Customer Support for review. You'll probably get a response either stating that the power is entirely Ranged and all defenses/bonuses applicable to its use should be handled the same as any other Ranged attack, or that the issue is open to DM interpretation (as all rules ultimately are) and they'll submit it to R&D.
–
IsziFeb 27 '11 at 8:00

No.

The power's description doesn't explicitly mention the Melee keyword for the sphere's attack, therefore that attack is not a Melee attack.

The only restriction is that the attack's target has to be adjacent to the sphere, but a close blast 1 or a [non-provoking] ranged 1 attack has the same effect. This means that the range restriction cannot be used to guess any implicit keywords of the attack in that way.

Compare the wording of these Conjuration powers, some of which explicitly mention the proxy's attack's type:

My reading is that - even if the attack listed in the spell block is not defined as a secondary attack - it complies to the same rules. Under this hypothesis, the attack inherits all the attributes of the encompassing power that are not explicitly overridden (see Rules Compendium, page 96 - Secondary Attacks).

Secondary Attacks

[...] Unless otherwise noted, the attack type and range of a secondary attack are the same as those of the power's primary attack, and the secondary attack doesn't require a separate action from the action of the primary attack.
[...]
Some secondary attacks have the same targets as their primary attacks, whereas others have targets of their own, noted in "Secondary Target" entries. [...]

A new, and broader, definition introduced by Rules Compendium is that of Secondary Powers (see page 97). It broadens the concept of secondary attack by defining an encompassing power (the one that you activate in the first instance) and secondary powers (a sort of temporary powers that you can use only while the encompassing one is active). The Warden's form powers fall gracefully in this category, but also do many Zone powers that hinder or damage creature for more than one round.

Secondary Powers

Some powers encompass what are called secondary powers. A creature must use the encompassing power to gain access to the secondary power. A secondary power requires a separate action from the action use to perform the encompassing power.
A secondary power's entry specifies its action type, attack/utility type, range and effects. [...]
Some secondary powers have the same targets as their encompassing powers, whereas others have targets of their own, noted in "Secondary Target" entries. [...]

Now, flaming sphere has no primary attack; however it clearly defines a range (20 squares). The ancillary attack has no range defined...
Also, even if the attack requires a separate standard action in rounds after the 1st (with the 1st being an exception), I don't feel right in electing it as secondary power because it lacks some of the information secondary powers provide.
This hints me at the concept of secondary (albeit hidden) attack.

So, I'd make the secondary attack inherit the range of the encompassing power but using its explicit targeting (1 adjacent creature).

This would make the flaming sphere attack a de facto Range 1 attack that doesn't benefit from targeting a prone creature.

This answer tries to infer the best we could from the rules and a power description that seems to lack a significant piece. This inference could be far away from what the power was designed for. I personally see no problem in explicitly considering the sphere attack as a melee one, as it has its good backing rationale.